1. Field of the Invention
The invention relates to optical fiber communications systems, in particular systems using plastic or silica fiber having typical core diameters of 100 to 250 xcexcm.
2. Discussion of the Related Art
Presently, the majority of optical fiber communication systems use silica-based fiber having typical core diameters of 62.5 xcexcm or less. For such cores, the alignment tolerances are very small, e.g., 10 xcexcm, when coupling a fiber to a device such as a transceiver. (Alignment tolerance indicates the allowable range of variation from the desired alignment position between device and fiber.) Thus, to achieve acceptable coupling efficiency, active alignment between the device and fiber is used. Active alignment involves, for example, placing a device and fiber adjacent one another, directing light from the device into the fiber, and measuring the output at the opposite end of the fiber. The relative positions of the device and/or fiber are adjusted until the output reaches acceptable levels, and then the coupling is secured, typically by a fast-curing adhesive or laser welding. Because such active alignment is complex, labor intensive, and expensive, passive alignment methods are preferred.
In passive alignment, a coupling structure is devised to provide adequate mechanical alignment between the device and fiber without any active alignment steps. Because passive alignment is usually feasible only for much higher alignment tolerances than found with typical silica-based telecommunications fiber, the technique is generally used only with large core fibers, e.g., 500 to 1000 xcexcm diameters. Moreover, because the only economically viable fibers with such large core diameters exhibit relatively poor loss and bandwidth characteristics, such passively-aligned couplings are typically found only in very short distance, low data rate systems, e.g., 50 meter links using 1 mm conventional plastic optical fiber at bit rates of 100 Mbits/second or less. Also, it is difficult to couple such large core fibers to high speed (1 Gbit/sec or more) detectors, since such detectors typically have an active area only 150 mm in diameter or less. In addition, attempts to extend passive alignment techniques to smaller-core fibers are complicated by the fact that optoelectronic device manufacturers anticipate use of active alignment. Specifically, the configuration of active devices within their packages, i.e., the exact location of the device with respect to the exterior of the package, is often inconsistent. For these numerous reasons, attempts to use inexpensive passive alignment have been largely restricted to fibers with core diameter greater than 500 xcexcm.
Recently, a new generation of relatively large core fibers with much higher bandwidths have been developed, including plastic optical fiber (generally graded-index perfluorinated fiber). These fibers have typical core diameters of 100 to 250 xcexcm. This core size allows for higher alignment tolerances than typical silica-based telecommunications fiber, but still makes the fiber compatible with Gbit/sec detectors. Thus, improved, low cost alignment techniques suitable for providing acceptable properties with fibers having core diameters of 100 to 250 xcexcm are desired.
The invention provides a system containing an adapter for passively aligning an active device such as a transceiver with an optical fiber having a 100 to 250 xcexcm core, such that the resulting coupling is within alignment tolerances, and provides desired coupling efficiencies and bit rates. In one embodiment, reflected in FIG. 1, the system contains a packaged active device 10 having a window or a lens 12, an optical fiber 14 having a core 16 of a diameter ranging from 100 to 250 xcexcm, and a fiber connector 18 attached to an end of the optical fiber such that a fiber endface 20 is provided. The system further contains an adapter 22 comprising a first receptacle 24 that secures the fiber connector and a second receptacle 26 that secures the packaged device, such that the device and the fiber endface are optically aligned. (Optically aligned indicates that light is able to travel from the fiber core to the device and vice versa.)
In a departure from the prior art, the exterior of the device package, in combination with the second receptacle, itself provides passive alignment within the adapter. And, in contrast to current passively-aligned systems, the system of the invention is able to achieve tolerances within 60 xcexcm, optionally within 35 xcexcm, i.e., the deviation from perfect alignment between the optical axis of the device and the center of the fiber core will be no more than 60 xcexcm, or no more than 35 xcexcm. Such tolerances make it possible to achieve, for example, a coupling efficiency of at least 50%, optionally at least 90%, in fiber having a 100 to 250 xcexcm diameter core. It is also possible to achieve bit rates of 1 Gbit/sec or greater, and even as high as 10 Gbit/sec for some applications.
The adapter is designed to securely hold the package and fiber connector in an arrangement within alignment tolerances. It is possible to design the adapter using design guidelines for thermoplastic resins, such that the device package is able to be snap-fit, e.g., by use of conventional cantilever snap joints using flexural snap-fitting arms or by forming the adapter from a relatively compliant or elastomeric material that plastically deforms upon insertion of the device package. Alternatively, it is possible to use adhesive to secure the package. It is also possible to form the adapter around all or a portion of the packaged device, e.g., by placing a portion of the package into a mold and then filling the mold with the adapter material (thereby making at least a portion of the package integral with the adapter). The receptacle for accepting a fiber connector is designed for the particular connector of interest, which is similarly capable of being snap-fit or adhered. In practice, the active device and adapter will generally be pre-assembled prior to installation of a system, and the fiber will then be connected during the actual installation.